The invention is related to cavity resonators, a method for producing a cavity resonator, and a band pass filter system comprising cavity resonators.
Cavity resonators are resonators where the radio frequency electromagnetic energy resonates in an empty volume, typically air or vacuum, this volume being surrounded by metal.
The size of a cavity resonator depends on the frequency of operation. At microwave frequencies (0.3 GHz-30 GHz), size and weight of cavity resonators are significant. They are typically milled in or cast from metal. Since the geometrical shape determines the frequency of resonance, high mechanical accuracy is required and/or post-production tuning is applied. Post-production tuning is usually achieved by placing a metallic tuning screw through the resonator wall, and turning it, causing suitable field distortion and thereby resonance frequency variation.
Cavity resonator filters combine several resonators in order to obtain sophisticated frequency selective behavior.
It turns out that cavity resonators and filters based thereof are large, weighty, expensive parts which are not well suited to mass production. They are nevertheless used because of their superior performance in terms of energy losses and high power handling capability.
A well known method to reduce the size of a cavity working at a given frequency is the shape of a “re-entrant cavity”, where the electric and magnetic parts of the electromagnetic field are essentially geometrically separated and the electric field volume is reduced in a capacitor. A tuning screw allowing resonance frequency correction is placed in the capacitive gap.
Cavity resonator filters made of metalized plastics may have advantages in terms of weight and cost. Metalized plastics cavity resonator filters have been used for surface-mount soldering onto printed circuit boards, thereby forming the cavity at one side by the surface metallization of the printed circuit board.
Problems occurring in the realization of surface-mount compatible cavity resonators and related filter systems are too high production tolerances (alignment, solder thickness) in the surface-mount and soldering process of the cavity parts and the printed circuit board.
Cavity filters consisting of a multitude of cavity resonators become inexpensive when produced in quantities, by using injection molded, metalized plastic parts. However, the molding form represents a big non-recurrent cost. Hence, all resonators of a given multi-resonator filter need to be based on the same molded parts.
A remaining drawback of this approach is that two or three different molded parts are actually required to build a resonator.
Another drawback of this above approach is that if an electronic tuning means (such as varactor diodes, MEMS devices) is to be placed onto the printed circuit board (PCB) inside the resonator, this device may be restricted in its impact as the geometrical position of the printed circuit board in the resonator may not be well suited for the tuning functionality.